Everytime a hero gets killed in action, he needs to wait a few seconds to be returned to the base. The process is called Respawning. While respawning, you can spectate other alive teammates or use one of the map cameras, chat, use Voice Comms or pings, or buy cards/attributes - but you can't use your dead hero.
Skilled players can use the fact that an enemy is dead to win a teamfight with a player advantage, or capture an objective.
The time it takes for you to respawn can be calculated from a formula:
Respawn Time in Seconds = 7 + (HeroLvl * 2 * (1 + Match Time Minutes * 0.03))